In conventional recording, data is written to a thin-film recording medium by a read-write transducer head which applies magnetic-flux signals to a high-coercivity thin film in a thin-film magnetic recording medium. The magnetic flux signals act to localize magnetic transitions in the medium, providing a stored form of the signals in the medium. In longitudinal recording, the read-write head has a longitudinally spaced gap for producing a magnetic flux in the direction of the plane of the medium, with the magnetic transitions in the medium being oriented also in the in-plane direction.
In the write mode, the oriented magnetic transitions provide localized magnetic fluxes which interact with a core in the read-write head, inducing a current in the core windings which can be amplified and "read" as a voltage-pulse signal. In a longitudinal recording, the flux lines in the oriented domains run substantially parallel to the plane of the medium.
In both recording and reproducing modes, flux losses occur which reduce the signal amplitude and storage density that can be achieved. One major source of flux loss is so-called spacing loss due to "uncaptured" flux in the spacing between the upper surface of the medium and the transducer head.
One approach to reducing spacing loss has been to reduce the distance between the surface of the medium and transducer head. This approach is limited by the problem of increased risk of collisions between the head and disc as the head is brought closer to the disc surface.
Another approach, disclosed in U.S. Pat. No. 5,041,992, uses a permeable magnetic layer, such as a permalloy layer, as a keeper layer to suppress spacing-loss flux. The keeper layer has a thickness between 300-1,000 .ANG.. The approach uses a bias on the transducer head, typically a DC bias, to saturate the region of the keeper between the poles of the transducer head, which lowers the permeability of this region, producing a magnetically defined virtual gap between the upper surface of the magnetic thin film and the transducer head. The net effect is that the spacing above the magnetic layer in which magnetic flux loss occurs is largely blocked in the region corresponding to the traveling region between the two poles in the transducer head.
One limitation with this approach is the requirement for a bias on the transducer head, to saturate the traveling inter-pole region in the keeper layer. The patent notes that in the absence of a bias flux, virtually the entire flux from a given magnetic transition will tend to be confined to the magnetic saturable layer, so that little if any flux reaches the transducer head. Not all read-write heads which are commonly employed in hard drive systems are designed for applying a bias.
A second limitation is that the permeable keeper layer tends to distort the magnetic flux field imposed on the medium during a write operation, causing some loss of overwrite properties and recording density, particularly in view of the greater distance between the transducer head and thin-film recording layer.
It would therefore be desirable to achieve reduced spacing loss in a recording system of this type, without the requirement for a biased transducer head, and without magnetic flux distortions produced by a keeper layer.